Conventionally, technologies for removing NOx in exhaust gas of an internal combustion engine include mainly a method using a three-way catalyst such as a Pt-Rh based catalyst, a selective reducing method using ammonia (JP-A-2003-286826), a selective reducing method using urea or the like (JP-A-2004-529286), a selective reducing method using hydrocarbon (HC) or the like, and a NOx adsorbing method for adsorbing NOx by various kinds of adsorbents (JP patent No. 3248187 corresponding to U.S. Pat. No. 6,367,246).
However, the method using a three-way catalyst does not have any performance of cleaning NOx for an exhaust gas containing a large amount of oxygen such as the exhaust gas of a diesel engine, a lean burn engine of low fuel consumption, or the like. In this manner, the method of using a three-way catalyst presents a problem that the three-way catalyst cannot exert its effect in an oxygen excess atmosphere and cannot remove NOx sufficiently.
Moreover, in the selective reducing method, circumstances are different depending on a reducing agent to be used. When ammonia or urea is used as a reducing agent, there are presented a problem that a tank for the reducing agent is required separately and a problem that a device is enlarged in size. Moreover, the reducing agent needs to be replenished, which lays a burden on a user. Further, there is presented a problem that ammonia is discharged into the atmosphere to cause secondary pollution (ammonia slip). Still further, the NOx adsorbing method presents a problem that NOx adsorbed by an adsorbent needs to be post-treated by rinsing with water or the like.
Consequently, to solve these problems, a copper ion exchange zeolite for catalytically cracking NOx directly into N2, O2 was developed and it was reported that a NOx cleaning rate higher than 90% could be obtained at an experiment stage. This copper ion-exchange zeolite is such that copper is carried on zeolite through ion exchange and has received attention as a reducing catalyst capable of cleaning NOx in an oxygen excess atmosphere and a catalyst device for cleaning exhaust gas having this copper ion-exchange zeolite set in the exhaust system of an engine is already publicly known (JP-A-5-68887).
However, the above-described catalyst device for cleaning exhaust gas does not have sufficient performance of cleaning HC and CO because the copper ion-exchange zeolite is low in the oxidizing ability of a catalyst in an oxygen excess atmosphere in a running automobile. For this reason, pretreatment is required to oxidize CH and CO, which becomes disadvantageous in terms of cost, space, and weight.
In contrast to these methods, in the selective reducing method using hydrocarbon as a reducing agent, a reducing agent can be obtained from the fuel of an internal combustion engine. Therefore, the selective reducing method using hydrocarbon as a reducing agent has received attention from the viewpoint of: eliminating the need for providing a separate tank, which is required in the method using ammonia and urea; reducing the size of a device; and load to be applied to a user. It is said that straight-chain alkane having 8 to 12 carbons is preferable as hydrocarbon used for the reducing agent of NOx (refer to non-patent documents 1,2).    [Non-patent document 1] Preprints of Catalysis Society of Japan, 1999 (3G20)    [Non-patent document 1] Preprints of Technology Conference No. 29-04, Society of Automotive Engineers of Japan
Moreover, oxygenated hydrocarbon containing oxygen such as alcohol, ethers, ketones, or the like is used as a reducing agent to reduce NOx and the like (JP-A-7-227523).
However, how to obtain a reducing agent required at the time of selectively reducing NOx in the exhaust gas and made of the above-described hydrocarbon from the fuel of the internal combustion engine becomes a problem. In addition, there is a demand for a technology that is plus in terms of energy balance and can reduce a total of NOx.